Hi Hartlepool,
Advice on planting your spuds from Sainsbury's is dont bother especially if you are going to plant in a greenhouse. I did the very same thing (ref Farmer Giles and potato virus) and the plants were heaving with some disease within a couple of weeks of the leaves appearing. Best to stick to the real seed potato from a good supplier.
Ref the greenhouse, I built a wood frame polythene house 10x13x9 feet high from fence posts and other cheap wood and covered by a poly tunnel plastic off-cut. The house works very well in a protected corner of the field but if I did it again I would give the roof more of a slope cos the condensation sticks to the inside roof and falls in showers when the wind blows. Cheap and nasty but it works well.
Best of luck from an ex pat Geordie in Ireland